Trench-type depressions (or deep trenches) are preferably used in the fabrication of memory cells. Since a charge that can be stored in a memory cell is proportional to the capacitance of the memory cell, there is a great deal of interest, particularly against the background of increasing miniaturization of microelectronic circuits, in keeping a capacitance of memory cells above a critical value even in the context of increasing miniaturization.
In semiconductor technology, patterning is effected by a combination of lithography with suitable etching methods. However, producing no longer only planar areas but also areas that are at an angle with respect to a reference area (the angle ranges through to 90° and beyond) gives rise to problems which can prevent patterning of such angled areas with lithography.
In order to solve this problem, the prior art has proposed methods which can be used for patterning in the vertical direction.
With regard to fabricating memory cells on the basis of providing trench-type depressions, it has been possible to deposit specific layer types such as, for example, an oxide only in a lower region of a trench, i.e. an electrode region, or only in an upper region of a trench, i.e. in a collar region.
By way of example, in a lower region of a trench-type depression processes have been carried out which relate inter alia to whole-area deposition of an oxide, filling of the structure with a suitable filling material, partial etching-back of the filling material, removal of an uncovered oxide and complete removal of the filling material.
In an upper region of a trench-type depression, the following have been realized: whole-area deposition of an etching stop layer, for example of a nitride layer, filling of a structure with a suitable filling material, for example polysilicon, partial etching-back of a filling material, removal of an uncovered nitride in an uncovered region, deposition or thermal generation of an oxide in an uncovered region, anisotropic etching-up of an oxide, removal of a filling material and removal of an etching stop layer. When suitable filling materials are used, it is possible to carry out the procedure described above without an etching stop layer.
With respect to such a depth patterning, it is furthermore possible, according to the prior art, to use an ALD (Atomic Layer Deposition) process for depth patterning. ALD processes are characterized in that at least two precursors are alternately admitted into a reaction chamber, a layer of defined thickness being deposited in a self-limiting manner in such a cycle. In this case, the deposition in vertical structures during a cycle is effected in a directional manner, beginning at the surface of the substrate. By limiting the precursor quantity supplied, it is thus possible to achieve a deposition only in an upper trench region of the structure. Typically, e.g. Al2O3 may be deposited in this way.
In a conventional manner, a deposition in a trench-type depression which does not take place uniformly in an upper region (a collar region) and a lower region of the trench-type depression is referred to as a “non-conformal” deposition. By means of such a non-conformal deposition, different materials can be deposited in the collar region produced by this deposition in comparison with the deeper regions.
In order to increase an electrode surface of capacitors that form a memory cell, grain elements are usually grown in the interior of a trench-type depression. With the use of different materials in the lower trench region and in the collar region, the grain elements produced in these regions may differ in form and size. Thus, by way of example, large grain elements are provided when using aluminum oxide (Al2O3) in the collar region in the case of a process of this type, while a grain size is reduced in a lower region of the trench-type depression.
A memory cell structure produced in accordance with a method according to the prior art is shown in FIG. 7. While the grain elements Ki formed in the lower region of the trench-type depression form an almost regular structure on the sidewalls of the trench-type depression, there is the risk that, on account of the increased grain size in the upper region of the trench-type depression, such large grain elements K2 will arise that the grain elements K2 will grow together, for example at a location identified by a reference symbol Z.
A principal disadvantage of methods according to the prior art is that the fabrication of a memory cell is made considerably more difficult by grains K2 growing together in such a way in the upper region of the trench-type depression.
Usually, a trench-type depression is etched into a silicon material (Si), a protective layer (pad nitride) being provided at the surface of the silicon wafer.